The architectures for today's Wireless Radio Access Networks (WRANs) are evolving towards a flatter, distributed structure, which will rely upon IP-based packet switching and Internet Engineering Task Force (IETF) protocols for data transport. In this regard, as voice services are moved from the traditional circuit-switched dedicated bearer model to an IP-based packet-switched model, Voice over IP (VoIP) will become a dominant application. However, notwithstanding the numerous advantages that may be realized with IP-based RANs, there are significant VoIP-related implementation issues that need to be resolved. For example, there are several unresolved issues related to the delivery of VoIP packets from a RAN to the Mobile Stations (MSs) involved. One of the most important of these issues involves determining how an IP-based RAN can support fast cell selection performed by MSs.
In future evolutions of the radio air interface protocols for IP-based RANs, the MSs will be capable of deciding which Base Transceiver Station (BTS) to receive data transmissions from. In that regard, FIG. 1 is a block diagram depicting the current centralized network architecture that has evolved. Referring to FIG. 1, the evolved centralized architecture 100 includes a core IP network 102 and an IP-based RAN 106 connected for IP-based communications via a gateway 104. The IP-based RAN 106 includes a Base Station Controller (BSC) 108 connected to the gateway 104 and a plurality of Base Transceiver Stations (BTSs) 110, 112 and 114. Each BTS 110, 112, 114 is connected to a plurality of MSs (e.g., as indicated by the connection between BTS 112 and MS 116).
In operation, the incoming and outgoing VoIP calls are anchored at a central BSC (e.g., BSC 108). Also, the BSC executes a Robust Header Compression (ROHC) IP-header compression scheme, and houses the Radio Link Protocol (RLP) instance for the MS involved. The BSC is responsible for making all of the resource allocations for the VoIP call, and the radio air interface's layer 3 signaling terminates in the BSC. The BSC is also responsible for maintaining the Active Set (i.e., adding or deleting sectors) for the MS involved, based on measurements of the strengths of the pilot signals transmitted by the MS.
FIG. 2 is a block diagram depicting an existing technique 200 used for fast cell selection in an IP-RAN. Referring to FIG. 2, as an MS (e.g., MS 116) engages in a VoIP call, MIP packets (including encoded voice packets) destined for the MS, arrive at the BSC (not shown). Before the BSC forwards the information to the MS, the BSC compresses the MIP header (and, possibly, other headers included within, such as UDP and RTP headers). In the radio air interface that has evolved, it will be possible for the MS to receive these packets from any BTS that supervises sectors in the MS's Active Set.
Referring to the existing fast cell selection technique 200 depicted in FIG. 2, the dashed line 202 shows that Forward Link (FL) data is sent to the MS (116) from one sector (BTS 114) in the MS's Active Set at a time. As shown by line 204, the MS indicates the BTS (BTS 112) from which the MS wants to receive data, by targeting the MS's Reverse Channel Quality Indicator Channel (R-CQICH) to the sector from which the MS wants to receive data, and setting the Desired Forward Link Serving Sector (DFLSS) bit to “1”. As shown by line 206, if the BTS (112) that controls that sector accepts the request, it returns a Forward Link Access Message (FLAM) to the MS, and the MS then begins to receive data on this new sector.
It is important to note that the signaling to be used for fast cell selection occurs at the Physical (PHY) and Medium Access Control (MAC) layers of the air interface. For increased speed and efficiency, it would be advantageous if the MS involved were able to indicate to the RAN which sector to use for the FL, without having to send layer 3 messages that must be processed at the BSC. For one thing, the PHY and MAC layers have all of the information needed to make the decisions involved, and sending layer 3 messages would require additional processing to send this information to the higher level layer for message creation purposes. Also, the signaling could be performed quicker and more efficiently if it were accomplished at the lower level PHY and MAC layers.
Essentially, the main implementation problem that needs to be resolved for fast cell selection in IP-based RANs is to determine how to synchronize the data transmissions at the various BTSs being controlled by a BSC, so that an MS can receive data from any BTS by using PHY and MAC level signaling. The existing synchronization approaches call for RAN signaling between the BSC and the BTSs, in order to activate bearer paths between the BSC and a new BTS after the new BTS receives the DFLSS data from the MS involved. This problem is illustrated by the existing fast cell selection RAN signaling technique shown in FIG. 3.
Referring to the existing fast cell selection RAN signaling technique 300 depicted in FIG. 3, lines 302a and 302b show that FL data is being sent to the MS (116) from one sector (BTS 114) in the MS's Active Set at a time. Line 304 shows that the MS (116) is indicating (with signaling) a new preferred FL serving sector (BTS 112). Line 306 shows that the new BTS (BTS 112) is requesting (with signaling) the BSC (BSC 108) to switch bearers, and line 308 shows that the BSC is acknowledging this request (with signaling). Line 310 shows that (with signaling) the BSC is informing the prior BTS (BTS 114) that the FL data transmissions are being terminated, and lines 312, 314 show that FL data is now flowing to the MS through the new BTS (BTS 112).
Unfortunately, the existing fast cell selection RAN signaling technique depicted in FIG. 3 will negatively impact delay-sensitive applications, such as VoIP. Consequently, an approach is needed for synchronizing the data transmissions of all BTSs in an Active Set, so that the FL transmissions can begin immediately after an MS selects a new serving sector, and also ensures that the transmitted packets will arrive in the proper order at the MS.